Mold additives for continuos casting of steel are added onto the surface of molten steel poured into a mold to form by receiving heat from the molten steel a layer structure above the molten steel surface, of a fused slag layer, a sintered layer and an unfused original mold additive layer, and then be consumed while gradually performing various duties. Its main role may be exemplified by the provision of:
(1) a lubricating action between the mold and a solidified shell;
(2) a melting and absorbing action of inclusions which float from inside of the molten steel; and
(3) a heat insulating action of the molten steel and the like are exemplified.
Recently, progress in continuous casting technology of steel has been remarkable and the demands on mold additives which have an influence on cast-piece qualities and operation stabilities have become even more strict, so that the quality of mold additives have been designed to accomodate various steel components and casting conditions.
Among the roles of mold additives (1) and (2) described above, are most important in controlling the characteristics of the mold additive such as softening point, viscosity, etc., so that selection of the chemical composition is important.
On the other hand, for heat insulation of the molten steel of (3), melting speed which is controlled by carbonaceous raw materials and powder characteristics such as bulk density, spreadability, etc. are important.
Even more recently, an exothermic type front mold additive in which molten steel temperatures at a meniscus portion in the mold are secured by improving (3) a step further and in order to improve the quality of castings, metal exothermic materials such as Ca-Si, Al, etc. are included in the mold additive to supply heat to the molten steel by generating exothermic reactions from oxidation in the mold, and then promptly fusing after the reaction to show the same behavior as a normal mold additive after fusing, has become desirable. Further, an exothermic type mold additive for the main has also been desired. Here, front mold additive means a mold additive which is used during irregular casting (at the beginning of casting, during tundish exchange) and main mold additive means a mold additive which is used during regular casting.
However, as it is necessary for an exothermic mold additive not only to obtain heat by exothermic reactions, but also to achieve the original duties of a mold additive after exothermic reactions as described above, various problems in quality design still remain.
When quality designing for a practical exothermic type mold additive for continuous casting, it is necessary to satisfy each of the following 3 items:
(i) that active additives not be contained in consideration of safety at the time of production, storage and use;
(ii) that exothermic reactions which can supply sufficient calorific value be obtained rapidly and uniformly without leaving unreacted substances, and that the calorific value, flame generating amounts, etc. can be controlled according to casting conditions when used; and
(iii) that exothermic reaction products rapidly form a fused glass layer, and be consumed by succesively flowing in between a mold and a solidified shell.
Up to now various exothermic mold additives have been proposed, however, there is no mold additive which satisfies the 3 items described above.
For instance, in Japanese Patent Laid Open No. 48-97735 a mold additive in which silicon, ferrosilicon and calcium-silicon are added as exothermic substances has been disclosed. This document discloses that these exothermic substances act as slag control agents on the one hand and that combustion heat can be obtained by the reaction with oxygen in the atmosphere on the other.
However, as metal powder which is added as an exothermic substance becomes an oxide for the first time by reacting in a solid or liquid state after being fused with oxygen in the atmosphere and then absorbed in the fused mold additive slag, various problems accur easily. Namely, at present, where gas blowing from a refractory for continuous casting has become common knowledge, as blown in gases such as argon, etc. enter into a mold and float into a mold additive, metal oxidation speed does not stabilize, so unreacted metal remains to be easily drawn into the fused mold additive slag or molten steel to obstruct the lubrication properties of the mold additive slag film. On the other hand, since this becomes a cause of quality deterioration of the castings as unreacted metal is picked up into steel to be the cause of inclusions and the like, it is not practical.
In Japanese Patent Laid Open Nos. 53-70039 and 58-154445, the addition of aluminum, aluminum alloys, calcium, and calcium alloys have been disclosed, however, as these additives include an active substance, they are not practical in view of (i) above.
Further, although Japanese Patent Publication No. 57-7211 proposes a mold additive in which a Ca-Si alloy is formulated, its exothermic reaction is not specifically described, but judging from its Examples, it is based on a method in which combustion heat is obtained by reaction of metal with oxygen in the atmosphere, it has drawbacks similar to the techniques described in Japanese Patent Laid Open No. 48-97735, so that it is not practical from the viewpoint of (ii) and (iii).
As the molten steel viscosity of so-called extremely low carbon steel having low carbon concentration, the production of which has been increasing in recent years, is high, the supply of heat to the meniscus in a mold can easily be insufficient and inclusions and gases which float from the inside of the molten steel can easily be caught by formation of an unsound solidified shell. As the captured inclusions and gases remain as defects in castings such as pin-holes, blow-holes, slag-bite and the like, scarfing becomes necessary and it not only becomes difficult to carry out hot charge rolling (hereinafter referred to as HCR) or hot direct rolling (hereinafter referred to as HDR), but also it becomes an obstacle when plastic processing of the latter process is carried out.
Therefore, in order to form a sound initial solidified shell which does not catch inclusions, it is necessary and indispensable to control temperature lowering of the meniscus inside the mold, and the insultating action of a mold additive becomes more important than that for conventional low carbon aluminum killed steel.
Further, in extremely low carbon steel, in a process after RH vacuum degassing treatment (Rheinstahl Huetten Werke & Heraus), it is necessary to control carburization and also to control carburization caused by a mold additive to the utmost. Therefore, although for mold additives it is desirable that the carbon content be small, just lowering the carbon content causes various problems. Carbonaceous raw materials are not only used as a slag melting speed control agent of a mold additive to control fused slag layer thickness but also contribute as a mutual sintering control agent for various raw materials in an unfused original mold additive layer together with maintaining a low thermal conductivity layer, keeping warm by exothermic reactions when they are oxidized. Therefore, if carbon content is simply decreased, it contributes to control carburization, but the thermal insulating property deteriorates, not only deteriorating casting quality but also adjustment of slagging melting speed becomes difficult, the thickness of a fused slag layer becomes too thick and sometimes it causes operational problems.
As described above, a mold additive for extremely low carbon steel which does not cause carburization and also has excellent thermal insulating properties is indispensable. However, it is presently true that a practical finished product has not been completed.
For instance, it has been disclosed in Japanese Patent Laid Open No. 64-66056 to use strong reducing substances such as metal, etc. in order to decrease carbon content to less than 1%. However, as oxidation exothermic reactions of added strong reducing substances depend on air oxidation and further because slagging speed is controlled thereby, under present conditions in which gas blowing from a refractory for continuous casting has become general knowledge, because argon gases enter into a mold to float to the surface, it is difficult to stabilize the oxidation speed of the strong reducing substances. Therefore, as stable exothermic reactions cannot be obtained and further, since unreacted additives remain and can be easily mixed into the fused mold additive slag or molten steel to obstruct the lubricating properties of the mold additive slag film, causing contamination of unreacted substances into the steel, becoming the origin of inclusions, etc. they accordingly become the cause of quality deterioration of castings so that they are not practical.